dev-plugins: Recommend cloning examples project instead of cookbook-plugin
dev-plugins: Recommend cloning examples project instead of cookbook-plugin

The cookbook-plugin is not a core plugin since version 2.14 and
is no longer maintained. It doesn't make sense to recommend it as
an example to follow for plugin development, since it is no longer
compatible with recent API versions.

Change-Id: Ic0fc04f5f18a699f1dfc8294d004c98945a8984d

qtproject/qtqa-gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,WANdisco/gerrit,WANdisco/gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit